Scientists pinpoint key ingredient in fighting pneumonia

A mysterious protein produced by a wide
spectrum of living things is crucial in regulating the immune response to the
most common form of pneumonia, a new Yale University School of Medicine study shows.
The study appears in Cell Host &
Microbe.

The protein—present in the blood of every
human being—is crucial to a successful immune system response to the bacteria
and also seems to prevent that response from damaging the host, according to
the researchers.

"We believe this may be a major cornerstone
in our understanding how we respond to many different kinds of pathogens," said
Jack A. Elias, chair of the Department of Internal Medicine and senior author
of the paper.

The Yale team investigated the protein,
which has a number of names including chitinase 3-like-1 (Chi3l1), because it
is a member of an ancient family of proteins found in species as diverse as
plants, insects, and humans. This evolutionary conservation led researchers to
believe that Chi3l1 plays an essential role in these organisms. However little
was known about the functions of this protein in health or disease.

The Yale team found that when mice lacking
this key protein were exposed to Streptococcus
pneumoniae, they experienced exaggerated levels of inflammation,
suffered much greater damage from inflammation and hemorrhage, and had a higher
rate of mortality and sepsis (blood poisoning) than those with the protein.
They found that the protein helped immune system cells called macrophages to
clear the pathogens from the body.

They also found that Chi311 inhibited the
innate immune response and decreased its ability to damage tissue in the host.
The latter is particularly important, note the researchers, because an
overwhelming innate immune system response contributes to the adverse
consequences of infections, such as respiratory failure and sepsis, which are
among the highest causes of death in hospitals.

"In recent years there has been a much
greater appreciation that the perfect immune system response is one that
eliminates bacteria but does not kill the host," said Charles Dela Cruz, assistant
professor of medicine and lead author of the paper. "This molecule seems to
help the organism achieve that balance."